In using the nonhuman primate as an auditory model for man, we hope to obtain a more precise understanding of the nature of hearing loss due to intense sound and noise and ototoxic drugs. We hope, for example, to be able to specify the conditions which produce broad band versus frequency specific hearing loss, either reversible or permanent in nature. We plan to examine more thoroughly the cumulative and combined effects of both drugs and noise. Particularly important is the relation between hearing impairment and cochlear and brain stem histopathology; in an attempt to clarify this relationship, we have continued working with Dr. J.E. Hawkins Jr., Dr. L-G. Johnsson, and Dr. Jens Hall (University of Oslo). Behavioral conditioning methods, together with audiometric testing procedures (i.e. Bekesy audiometry) are used for threshold testing, for frequency and intensity difference thresholds, and for assessment of the discrimination of loudness levels in the study of recruitment. Monkeys with a clearly established hearing loss as a consequence of exposure to intense sound or drug treatment are sacrificed and histological changes in the cochlea and cochlear nucleus of the brain stem are compared with the final audiometric results.